It is wrong to describe only the two possibilities:
- The game is complex and you need to read a complicated manual in order to use it or
- The game is so streamlined that you don't need a manual.

There is a third possibility:

Today's software technology easily allows to have all information you need available at a (right) mouse click, which were printed in manuals in old days.

The additional advantage is, that it is context-sensitive. So if you right click on an attribute in you character sheet, for example, it could explain, for which classes this attribute is important, how it influences e. g. damage values etc.

Drakensang for example does this quite good: If you right click on your attack value e. g., you see the exact formula, how it is computed from attributes etc.
(Still some info is missing, but as an approach I like it.)

So for me the rule is clear: Things, which can be integrated into the GUI due to today's technology, should be. Having to read them in a manual instead, is simply
unnecessary work.

Remark:It is like with in game maps: Hand holding, where the in-game map shows you the location of the next quest step, if you cannot know it in game, is bad. But if you talk to a character and he tells you a location, than it is completely ok to see that location on the in game map afterwards, because you could also write it down on a piece of paper.

So my rule is: The computer should spare the player from all mechanical work, which can be better done by the computer. It should not take away the thinking.

Of course there are other reasons to have a manual, like lore, having a handbook of a rule set etc. So the ideal world for me is to have everything in a handbook and in addition have in game help etc.

For example the complete TDE rulebook, which accompanied the German Drakensang version (in electronic form, but you can print it, if you like) was great.

Dumbing down games has nothing to do with the need to avoid a manual. Dumbing down games comes from dumbed down players.